1. Field of the Invention
This invention relates generally to remote sensing devices, and more particularly to a circuit which compensates for lead wire resistance to provide a linear voltage output when coupled to a remote resistive sensing element.
2. Description of the Prior Art
In devices such as strain gauges and resistive temperature measuring devices, it is often necessary to employ a resistive sensing device remotely located from its associated instrumentation circuits.
Strain gauges are devices which rely on changes in electrical resistance when the resistive measuring element is subjected to forces of tension or compression. Such devices are used in scales, pressure sensors, vibration monitors, etc. The resistive element or network of elements is typically mounted in an enclosure which is in turn mounted on or near the device or area to be monitored. Two or more lead wires from the resistive elements are coupled to an electronic installation which may be located as much as thousands of feet from the sensing elements. If wire of a reasonable size (e.g., 22-20 wire) is employed, the lead wire resistance soon becomes appreciable when compared to that of the sensor which is typically 50-500 ohms.
It is usually necessary to use a bridge of four elements mounted at the sensing point. If four elements are not required by the particular application, bridge completion resistors are contained within the device. The bridge is excited by a current source via a first pair of wires, and the output voltage is returned over a second pair of wires. A third pair of wires may be used for remote voltage sensing and control. Current sources or voltage sensing are required due to voltage drops caused by the lead wire resistances.
Temperature sensing using platinum wire or nickel alloy temperature sensing elements is well known and is considered the most accurate commonly used method of industrial temperature sensing. The platinum wire element typically has a resistance of only 100 ohms at room temperature while the nickel alloy element may have a resistance of 1000 ohms. While the nickel alloy element is therefore easier to instrument, the platinum wire element is preferred for its accuracy and long term stability.
Temperature sensing elements do not usually employ bridge completion resistors which makes instrumentation for automatic read-out especially difficult. Manual read-out is possible using a potentiometer and galvanometer with three lead wires. While such an arrangement yields a linear output with no errors due to lead wire voltage drops, this arrangement is not suitable for continuous automatic read-out which is becoming increasingly important with the increased use of microprocessors in automatic control systems.
U.S. Pat. No. 3,817,104 describes a temperature measuring voltage to current converter including a temperature-responsive resistance bridge. However, the apparatus taught in this patent includes no means for compensating for lead wire resistance. Further, the apparatus produces a non-linear response due to the use of the resistive bridge.
U.S. Pats. Nos. 4,000,454 and 4,060,715 address the problem of linearizing the non-linearity of the sensing element; however, the arrangements taught include no means for accomplishing lead wire resistance compensation.
Finally, U.S. Pat. No. 3,924,470 teaches a temperature measurement circuit employing three lead wires which in itself offers some relief from the problem of lead wire resistance. However, such relief extends only to short lead wires resulting in small changes in the resistance of the resistive sensing element.